Method of making aluminum-coated metal

ABSTRACT

A method of coating a metal substrate is disclosed in which the metal substrate is disposed as an anode in an electrolytic circuit with an aqueous alkaline liquid electrolyte. An electrical voltage differential is applied to the metal substrate and a cathode. The treated metal substrate is disposed as a cathode in an electrolytic circuit with an ionic liquid electrolyte and an aluminum salt. An electrical voltage differential is applied to the metal substrate and an anode to form an aluminum layer on the metal substrate.

This invention was made with Government support under contract numberW911W6-12-2-0003 awarded by the United States Army. The Government hascertain rights in the invention.

BACKGROUND

Metal plating such as cadmium plating has been widely used on variousmaterials, including but not limited to high-strength steel, aircraftcomponents, fasteners, electrical connectors, and numerous others. Metalplating can be used to promote properties such as corrosion resistance,lubricity, and electrical conductivity. Cadmium has been used as a metalplating material for various reasons such as the cost of plating and itsanti-galling performance (no adhesive wear on threaded surfaces).However, continued commercial uses of cadmium have been facing pressuredue to health concerns in recent years, including being listed as asubstance of very high concern (SVHC) in 2012 by the European Unionenvironment & safety regulatory agency REACH.

Various metal plating technologies have been developed and evaluatedover the years. For example, aluminum has been proposed as a metalplating material, and has been approved for corrosion protection ofhigh-strength structural steels in aircraft as set forth inspecifications such as MIL-DTL-83488. However, various technical issuescontinue to present challenges. Commercially available aluminum metalplating processes utilize toxic or environmentally unfriendly organicsolvents and pyrophoric alkylaluminum compounds that require extrameasures for operation under inert conditions. Therefore, therecontinues to be a need for further developments regarding aluminum metalcoatings.

High strength steels (HSS) are used throughout aerospace industries forcomponents such as aircraft landing gear, bolts, aircraft tail hooks onaircraft carriers etc. High strength steels are protected by sacrificialcoatings such as electroplated cadmium, zinc-nickel and non-electrolyticaluminium-based SermeTel (Praxair Surface Technologies) coatings. It hasbeen known that HSS are prone to hydrogen embrittlement representingbrittle mechanical failures under stress that is manifested by brittleintergranular fracture and trans-granular cleavage. Incorporation ofhydrogen during either the electroplating process or the corrosionduring service promotes the increased risk of failure arising fromhydrogen embrittlement. Porous electroplated coatings can allow hydrogento be released from the steel substrates via a post-plating bakingprocess. However, substrates coated with porous coatings can becomesusceptible to re-embrittlement due to galvanic cells formed on exposedarea. Dense coatings of lower porosity can mitigate the risk of hydrogenre-embrittlement; however, dense coatings can prevent hydrogen frombeing released. Hydrogen embrittlement can also be caused bypre-treatment processes for of HSS, such as treatment with acids orelectrolytic cathodic cleaning which can generate hydrogen.

BRIEF DESCRIPTION

According to some embodiments, a method of coating a metal substratecomprises disposing the metal substrate as an anode in an electrolyticcircuit comprising an aqueous alkaline liquid electrolyte. An electricalvoltage differential is applied to the metal substrate and a cathode.The treated metal substrate is disposed as a cathode in an electrolyticcircuit comprising a liquid electrolyte that comprises an ionic liquidand an aluminum salt. An electrical voltage differential is applied tothe metal substrate and an anode to form a layer comprising aluminum onthe metal substrate.

According to some embodiments, an article comprises a metal substrateand a layer thereon comprising aluminum. The layer comprising aluminumhas a volumetric density greater than 99%, and the coated article meetsor exceeds hydrogen embrittlement test requirements of ASTM F519-10.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of this disclosure is particularly pointed out anddistinctly claimed in the claims at the conclusion of the specification.The foregoing and other features, and advantages of the presentdisclosure are apparent from the following detailed description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a schematic depiction of a cross-sectional view of a coatedarticle as described herein;

FIG. 2 is a schematic depiction of an example embodiment of a method asdescribed herein; and

FIG. 3 is a schematic depiction of another example embodiment of amethod as described herein.

DETAILED DESCRIPTION

With reference now to the Figures, FIG. 1 schematically depicts across-sectional view of a coated article 10 comprising a substrate 12having thereon a surface layer 14 comprising aluminum. The substrate 12can comprise high strength steels that are particularly prone tohydrogen embrittlement due to lack of corrosion resistance and facilehydrogen diffusion in the steel lattice. Further, the substrate 12comprises a metal or metal alloy having a first oxidation-reductionequilibrium (corrosion) potential more noble than aluminum and aluminumalloys. This difference in potential can allow the layer to providesacrificial corrosion protection in an electrolyte that is experiencedin the service environment where the parts are used. In someembodiments, the metal or metal alloy of substrate 12 has an electrodepotential of −1.0 V to 0 V with respect to saturated calomel electrode(SCE) in 3.5% wt. sodium chloride solution or sea water solution orsynthetic sea water solution, more specifically −0.6 to −0.1, and evenmore specifically from −0.4 V to −0.2 V. Examples of metals or metalalloys for the substrate 12 include but are not limited to high-strengthsteels (e.g., D6AC, 300M, M-50, AERMET 100, 4330, 4340, 52100). In someembodiments, the substrate 12 comprises a high strength low alloy (HSLA)steel as that term is defined by SAE (Society of Automotive Engineers)standards.

The thickness of layer 14 can be specified to meet targetspecifications. In some embodiments, the layer 14 has a thickness in arange having a low end of 2.5 μm (0.0001 inches), more specifically 7.5μm (0.0003 inches), and even more specifically 12.5 μm (0.0005 inches),and an upper end of 17.5 μm (0.0007 inches), more specifically 25 μm(0.001 inches), and even more specifically 250 μm (0.010 inches). Theabove range endpoints can be independently combined to serve as adisclosure of a number of different ranges, which are hereby expresslydisclosed. In some embodiments, the layer 14 can be utilized to promoteresistance to corrosion. In some embodiments, the layer 14 can beutilized to promote resistance to galling along contact portions of thearticle. In some embodiments, the layer 14 can be disposed on surface tobe subjected to sliding contact with another article or component. Sucharticles can include, but are not limited to, threaded fasteners,press-fit connections, propeller barrels, electrical connectors,press-fit high strength steel bolts used in turboprop propellers, andother various fasteners or connectors.

In some example embodiments, articles such as shown in FIG. 1 can beprepared by methods shown in FIGS. 2 and 3. In some embodiments, thesurface of the substrate 12 can optionally be subjected to physicalsurface preparation such as degreasing, grit blasting, or bothdegreasing and grit blasting as shown in FIGS. 2 and 3. Examples ofdegreasing agents include organic solvents such as acetone or methylethyl ketone, alkaline degreasers such as aqueous solutions of sodiumhydroxide, potassium hydroxide, or ammonia, or aqueous surfactants withdegreasing capability. After degreasing, the substrate can optionally bedried such as by radiant heat or blowing with air. Grit blasting can beperformed by directing abrasive particles at the substrate at velocitiesof 50 to 100 in/min. Examples of materials for grit blasting includealumina grit, silicon carbide grit, steel grit, or glass particles. Theparticles for grit blasting can have particle sizes expressed as meandiameter and distribution as described in ANSI B74.12-2001, thedisclosure of which is incorporated herein by reference.

As further shown in FIGS. 2 and 3, after optional degreasing and gritblasting, the substrate is subjected to anodic alkaline electrolytictreatment. This treatment involves disposing the metal substrate as ananode in an electrolytic circuit comprising an aqueous alkaline liquidelectrolyte, and applying an electrical voltage differential between themetal substrate anode and a cathode. Conditions for anodic alkalineelectrolytic treatment can be controlled to promote formation of oxygenat the metal substrate surface according to the reaction 4OH⁻→2H₂O+O₂and to minimize formation of metal oxide at the metal surface substrate.In some embodiments, the formation of oxygen at the metal substratesurface can promote cleaning of the metal substrate surface. Thealkaline electrolyte can comprise aqueous solutions of alkalinecompounds such as sodium hydroxide, potassium hydroxide, and their weakacid salts such as Na₂CO3, Na₃PO₄, Na₂SiO₄ at concentrations in a rangehaving a lower end of 15 wt. %, 10 wt. %, or 5 wt. %, and an upper endof 50 wt. %, 75 wt. %, or 100 wt. %. Commercial alkaline cleaningproducts such as Henkel Turco 4181 can be used as well. The above rangeendpoints can be independently combined to serve as a disclosure of anumber of different ranges, each of which is hereby expressly disclosed.The alkaline electrolyte can have a pH in a range having a lower end of9, 8, or 7.5, and an upper end of 14. The above range endpoints can beindependently combined to serve as a disclosure of a number of differentranges, each of which is hereby expressly disclosed. The alkalineelectrolyte can be at a temperature in a range having a lower end of 25°C. and an upper end of 50° C. In some embodiments, the voltagedifferential applied between the cathode and the metal substrate anodecan provide an electrical current density (based on anode surface area)between the cathode and anode in a range having a lower end of 5 mA/cm²,10 mA/cm², or 15 mA/cm², and an upper end of 25 mA/cm², 50 mA/cm², or 75mA/cm², and can be applied for a duration in a range having a lower endof 15 seconds, 30 seconds, or 60 seconds, and an upper end of 120seconds, 180 seconds, or 300 seconds. The above range endpoints can beindependently combined to serve as a disclosure of a number of differentranges, each of which is hereby expressly disclosed. In someembodiments, the current can be pulsed or varied. Other conditions(e.g., temperature) can also be varied during anodic alkalineelectrolytic treatment. After anodic alkaline electrolytic treatment,the metal substrate can be rinsed with deionized water and dried innitrogen or air.

With reference now to FIG. 3, the anodic alkalineelectrolytically-treated metal substrate can be further subjected to ananodic ionic liquid electrolytic treatment. This treatment involvesdisposing the metal substrate as an anode in an electrolytic circuitcomprising a liquid electrolyte that comprises an ionic liquid, andapplying an electrical voltage differential between the metal substrateanode and a cathode to induce current. In some embodiments, thistreatment can etch the surface of the metal substrate, which can promoterelease of metal oxide from the surface. The ionic liquid electrolytecan be at a temperature in a range having a lower end of 25° C., 40° C.,or 55° C., and an upper end of 60° C., 70° C., or 80° C. The above rangeendpoints can be independently combined to serve as a disclosure of anumber of different ranges, each of which is hereby expressly disclosed.In some embodiments, the voltage differential applied between thecathode and the metal substrate anode can be set to produce a currentdensity (based on anode surface area) in a range having a lower end of 5mA/cm², 10 mA/cm², or 15 mA/cm², and an upper end of 20 mA/cm², 30mA/cm², or 40 mA/cm², and can be applied for a duration in a rangehaving a lower end of 15 seconds, 30 seconds, or 45 seconds, and anupper end of 60 seconds, 120 seconds, or 300 seconds. The above rangeendpoints can be independently combined to serve as a disclosure of anumber of different ranges, each of which is hereby expressly disclosed.In some embodiments, the voltage can be pulsed or varied. Otherconditions (e.g., temperature) can also be varied during anodic ionicliquid electrolytic treatment. After anodic ionic liquid electrolytictreatment, the metal substrate can be rinsed (e.g., with methanol,ethanol, deionized water, or a combination of rinsing agents) and dried(e.g., with heat or blown air), or can remain in the ionic liquidelectrolyte for further processing.

As further shown in FIG. 3, the anodic ionic liquidelectrolytically-treated metal substrate can be subjected to sonicatingtreatment in the same or a different electrolyte bath. In someembodiments, anodic etching treatment of steel substrates in ionicliquids can produce smut on the substrate, which can interfere with thesubsequent aluminum plating process, leading to poor coating quality.Sonicating the parts can be conducted in ionic liquids or in water-freeorganic solvents such as ethanol, toluene, acetone, etc. In someembodiments, sonic treatment can promote the removal of smut or othersubstances that may have been formed on the surface by the treatmentprocess or are otherwise disposed on the surface. In some embodiments,sonicating treatment can involve exposure of the metal substrate tosonic energy in a frequency range having a lower end of 10 kHz and anupper end of 200 kHz. In some embodiments, the sonic treatment can beapplied for a duration in a range having a lower end of 30 seconds andan upper end of 300 seconds. In some embodiments, the sonic energy canbe pulsed or varied.

As further shown in FIGS. 2 and 3, a layer comprising aluminum can bedeposited onto the treated metal substrate by disposing the metalsubstrate as a cathode in an electrolytic circuit comprising a liquidelectrolyte that comprises an ionic liquid and an aluminum salt, andapplying an electrical voltage differential between the metal substrateand an anode. As used herein, the term “ionic liquid” means a salthaving a melting point below the processing temperature (e.g., below100° C.). In some embodiments, the ionic liquid is non-volatile or oflow volatility at the process temperature. Cations for the ionic liquidused as electrolyte for cathodic electrodeposition of aluminum shown inFIGS. 2 and 3, as well as for the anodic electrolytic treatment shown inFIG. 3, can include, but are not limited to imidazolium (e.g.,1-ethyl-3-methylimidazolium, 1-ethyl-2,3-dimethylimidazolium,1-butyl-3-methylimidazolium (“BMI”), 1-hexyl-3-methyl-imidazolium(“HMI”), pyridinium (e.g., N-methylpyridinium), tetraalkylammonium,pyrrolidinium (e.g., 1-butyl-1-methyl-pyrrolidinium (“BMPyr”),trialkylsulfonium (e.g., triethylsulfonium), pyrazolium, triazolium,thiazolium, oxazolium, pyridazinium, pyrimidinium, pyrazinium. Exemplaryanions for ionic liquids used in the embodiments described hereininclude, but are not limited to, chloroaluminate (Al₂Cl₇ ⁻),tetrafluoroborate (BF₄), hexafluorophosphate (PF₆),trifluoromethanesulfonate (CF₃SO₃), bis(trifluoromethylsulfonyl)imide,trifluoroethanoate, nitrate, SCN, HSO₄, HCO₃, CH₃SO₃, CH₃CH₂SO₄,(CH₃(CH₂)₃O)₂POO, (CF₃SO₂)₂N, dicyanamide, (CF₃CF₂SO₂)₂N, L-(+)-lactate,CH₃SO₄, and CH₃COO, and the like. In some embodiments, the ionic liquidhas a cation that is an imidazolium, and more specifically the ionicliquid has the formula:

wherein, R and R₁ are independently selected from H, an unsubstituted orsubstituted alkyl group having 1 to 30 carbon atoms, or an unsubstitutedor substituted aryl group having 6 to 30 carbon atoms. X is an anionicgroup, as described hereinabove, that associates with imidazolium toform an ionic-liquid cation/anion pair.

In addition to the cation (or mixtures of cations) and anion (ormixtures of anions) of the ionic liquid, the liquid electrodepositioncomposition for depositing the layer comprising aluminum also comprisesan aluminum salt. Aluminum salt can be introduced to the composition inthe form of aluminum chloride (AlCl₃), but will tend to form differentaluminum-containing ions in the ionic liquid composition, including butnot limited to AlCl₄ ⁻ (tetrachloroaluminate) or Al₂Cl₇ ⁻(heptachlorodialuminate). Aluminum-containing anions can also beintroduced electrolytically by electrochemical reaction of metallicaluminum in the anode(s) of an electrochemical cell of which theelectrodeposition forms a part. The electrodeposition composition canalso include additives to improve the integrity of the aluminum coatingsuch as a nucleation aid like a surfactant. Other additives are known inthe art, see, e.g., US 2012/0006688 A1, the disclosure of which isincorporated herein by reference in its entirety, and can be included aswell. Organic solvents can also be present in amounts up to 30 wt. %,such as toluene, chlorobenzene, dichlorobenzenes, xylene, cyclohexane,heptane, and others.

In some embodiments, process conditions for deposition of the layercomprising aluminum can be managed to promote targeted results such asdensity of the aluminum layer as described in more detail below. Inpractice, an electrical current is provided by a power source that issufficient to provide an electric current density (current per effectiveelectrode area) of at least 5 mA/cm², more specifically of at least 10mA/m², even more specifically at least 20 mA/cm², and even morespecifically at least 30 mA/cm². Current is applied until the desiredaluminum coating layer thickness is achieved (e.g., 5 μm to 50 μm). Theelectrodeposition method can be carried out at temperatures ranging from20° C. to 100° C., more specifically from 20° C. to 80° C., and evenmore specifically from 60° C. to 70° C. The above range endpoints can beindependently combined to serve as a disclosure of a number of differentranges, each of which is hereby expressly disclosed.

In some embodiments, the above-described methods can avoid conditionsthat can contribute to hydrogen embrittlement of the substrate 12. Forexample, the above-described electrolytic treatment of the substrate 12as an anode in aqueous alkaline electrolyte can evolve oxygen at thesurface of the metal substrate that can promote cleaning of the surfacebefore electrolytic deposition of aluminum in an ionic liquidelectrolyte. Cathodic electrocleaning in an acidic electrolyte, on theother hand, would evolve hydrogen at the surface of the metal substratethat could contribute to the promotion of hydrogen embrittlement of thesubstrate 12. Hydrogen embrittlement of the substrate 12 can be managedby post-fabrication heat treatment to drive hydrogen out of thesubstrate, but the effectiveness of such post-fabrication heat treatmentcan be limited in the case of dense aluminum overlayers, which caninhibit removal of hydrogen from the substrate. In some embodiments, theabove-described methods can avoid conditions that contribute to hydrogenembrittlement, thereby allowing for deposition of higher densityaluminum layers (i.e., volumetric density greater than 99%). Unlikecadmium and zinc-nickel plating on high strength steels which wouldrequire a baking process at an elevated temperature (ca. 200° C.) toliberate hydrogen absorbed in the metal lattice, the methods describedherein don't require baking after the coating is applied to thesubstrate. As mentioned above, in some embodiments, the coated articlemeets or exceeds requirements of the hydrogen embrittlement test asprescribed in ASTM F519-10. As used herein, the coated article meets orexceeds these requirements if the article passes the sustained load testof ASTM F519-10 of greater than 200 hours at 75% of the notch fracturestrength.

In some embodiments, the layer 14 can be treated with a trivalentchromium passivation process. Such a process can be carried out bytreatment of the layered substrate (e.g., by dipping or application witha brush, sponge, spray, or other coating applicator) with an aqueoussolution or non-aqueous solution comprising trivalent chromium andvarious anions. Exemplary anions include nitrate, sulfate, phosphate,and/or acetate. Specific exemplary trivalent chromium salts can includeCr₂(SO₄)₃, (NH)₄Cr(SO₄)₂, KCr(SO₄)₂, CrF₃Cr(NO3)₃, and mixturescomprising any of the foregoing. Embodiments of compositions and theapplication thereof to substrates are described in U.S. Pat. Nos.5,304,257, 5,374,347, 6,375,726, 6,511,532, 6,521,029, and 6,511,532,the disclosures of which are incorporated herein by reference. Variousadditives and other materials can be included in the compositioncomprising trivalent chromium as disclosed in the patent literature, andthe trivalent chromium salt composition can be selected from any of anumber of known commercially-available compositions.

While the present disclosure has been described in detail in connectionwith only a limited number of embodiments, it should be readilyunderstood that the present disclosure is not limited to such disclosedembodiments. Rather, the present disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the present disclosure.Additionally, while various embodiments of the present disclosure havebeen described, it is to be understood that aspects of the presentdisclosure may include only some of the described embodiments.Accordingly, the present disclosure is not to be seen as limited by theforegoing description, but is only limited by the scope of the appendedclaims.

1. A method of coating a metal substrate, comprising disposing the metalsubstrate as an anode in an electrolytic circuit comprising an aqueousalkaline liquid electrolyte, and applying an electrical voltagedifferential to the metal substrate and a cathode; and disposing themetal substrate as a cathode in an electrolytic circuit comprising aliquid electrolyte that comprises an ionic liquid and an aluminum salt,and applying an electrical voltage differential to the metal substrateand an anode to form a layer comprising aluminum on the metal substrate.2. The method of claim 1, further comprising degreasing the metalsubstrate prior to disposing in the aqueous alkaline liquid electrolyte.3. The method of claim 1, further comprising grit blasting the metalsubstrate prior to disposing in the aqueous alkaline liquid electrolyte.4. The method of claim 1, further comprising degreasing and gritblasting the metal substrate prior to disposing in the aqueous alkalineliquid electrolyte.
 5. The method of claim 1, further comprisingsubjecting the metal substrate to ultrasonic cleaning prior to formingthe layer comprising aluminum on the metal substrate.
 6. The method ofclaim 1, wherein the formation of the layer comprising aluminum on themetal substrate is performed under conditions to produce the layercomprising aluminum, at a volumetric density greater than 99%.
 7. Themethod of claim 1, wherein the metal substrate comprises steel.
 8. Themethod of claim 1, wherein the electrolytic circuit comprising a liquidelectrolyte that comprises an ionic liquid and an aluminum salt includesan aluminum anode.
 9. The method of claim 1, further comprising, priorto forming the layer comprising aluminum on the metal substrate,disposing the metal substrate as an anode in an electrolytic circuitcomprising a liquid electrolyte that comprises an ionic liquid, andapplying an electrical voltage differential to the metal substrate and acathode.
 10. The method of claim 9, further comprising degreasing andgrit blasting the metal substrate prior to disposing in the aqueousalkaline liquid electrolyte.
 11. The method of claim 9, furthercomprising subjecting the metal substrate to ultrasonic cleaning priorto forming the layer comprising aluminum over the metal substrate. 12.The method of claim 9, wherein the formation of the layer comprisingaluminum on the metal substrate is performed under conditions to producethe layer comprising aluminum, at a volumetric density greater than 99%.13. The method of claim 9, wherein the metal substrate comprises steel.14. The method of claim 9, wherein the electrolytic circuit comprising aliquid electrolyte that comprises an ionic liquid and an aluminum saltincludes an aluminum anode.
 15. An article, comprising: a metalsubstrate; and a layer comprising aluminum over the metal substrate, ata volumetric density greater than 99%, wherein said article meets orexceeds requirements hydrogen embrittlement test requirements of ASTMF519-10.
 16. The article of claim 15, wherein the metal substratecomprises steel.
 17. The article of claim 16, wherein the metalsubstrate comprises high strength low alloy steel.